In general, a safety valve is a pressure relief device that is operated automatically by positive pressure on an inlet port side and is characterized by instantaneous opening. Such a safety valve typically has a built-in spring, and is opened or closed by force acting on this spring. This safety valve is used to prevent overpressure or to keep pressure constant in an apparatus that handles fluid.
Particularly, a pressurizer safety valve used for a reactor coolant system of a nuclear power plant is a very important component for preventing overpressure of the reactor facility. Thus, this pressurizer safety valve is periodically tested in order to check whether it is accurately operated within a set pressure tolerance.
The test of the pressurizer safety valve is specified in in-service inspection code and standards such as American Society for Mechanical Engineering (ASME) OM App. I, ASME PTC 25, Korea Electric Power Industry Code (KEPIC) MOD, KEPIC MBK, and so on.
Based on these code and standards, the pressurizer safety valve undergoes a set pressure test once every five (5) years. At least 20% of all safety valves are subjected to the set pressure test every two (2) years. When a part of the safety valve is replaced, the replaced safety valve undergoes the set pressure test before production of electric power is resumed.
The test methods complying with these code and standards include an in-situ test using an auxiliary lift device with the safety valve installed on the facility, a bench test using a bench set on which the safety valve is mounted, and a test facility test using an off-site test facility.
The in-situ test is a method that measures the set pressure with the safety valve installed on the facility. Particularly, if the safety valve is installed on the facility, for instance, by welding, only the in-situ test can be used.
The auxiliary lift device is a device that applies an auxiliary lifting force to system pressure in order to open the safety valve. For the purpose of the set pressure test, the auxiliary lift device is mounted on the safety valve. After keeping thermal equilibrium for 30 minutes or more, the pressure of an air motor of the auxiliary lift device is gradually increased to the set pressure of the safety valve so as to pop open the safety valve.
The set pressure test of the pressurizer safety valve using the auxiliary lift device is generally performed using the auxiliary lift device prior to a cool down operation after the reactor is stopped in order to minimize an influence on the power plant operation during testing and to determine whether or not the pressurizer safety valve requires maintenance during overhaul of the power plant. When this test is performed, there is a possibility of causing a transient phenomenon in which the pressure of a reactor coolant system is reduced during operation, and a fluid containing radioactive materials is discharged through the safety valve.
When the safety valve is equipped with a loop seal, if the set pressure test is performed in the state in which loop seal water is drained, it is impossible to measure an influence of the operation of the safety valve associated with discharge of water from the loop seal. In contrast, when the set pressure test is performed in the state in which loop seal water is not drained, parts of the safety valve may be damaged or loosened by vibration due to instantaneous discharge of water from the loop seal when the safety valve is operated.
The bench test is a method of testing the safety valve attached to the bench set having a flange at room temperature. This bench set is equipped with a pressure supply apparatus of a fluid such as nitrogen, air or water, a pressure controller, a flange coupled with the subject safety valve, a pressure gauge connected to the flange, and so on. If necessary, the bench set may include a complicated device having several flanges coupled with, for instance, a chart recorder for recording set values in order to perform various functions and an automatic test.
The type of this bench set is determined according to the number and kind of safety valves to be tested, code requirements, and available space. Examples of the bench set are well disclosed in Korean Patents Nos. 10-0540308 (titled Apparatus for Testing Seat Tightness and Pressure Setting of Pressurizer Safety Valves granted on Dec. 26, 2005) and 10-0311775 (titled Safety Valve Testing Apparatus granted on Sep. 28, 2001), and U.S. Pat. No. 4,893,494 (titled Method and System for Testing Safety Relief Valves granted on Jan. 16, 1990).
The apparatus for testing seat tightness and pressure setting of pressurizer safety valves disclosed in Korean Patent No. 10-0540308 is designed to directly carry out a seat tightness test and a pressure setting test on the pressurizer safety valve at room temperature. The apparatus for testing seat tightness and pressure setting of pressurizer safety valves, each of which has a seat tightness checking flange, includes: a nitrogen gas storage storing nitrogen gas; a pump connected to the nitrogen gas storage, pressurizing and pumping the nitrogen gas, pressure of which is increased to seat tightness test or pressure setting test values of the safety valve, and discharging the pumped nitrogen gas to a pressure storage tank; the pressure storage tank storing the nitrogen gas discharged from the pump; a pressure regulating valve connected to the pressure storage tank, regulating pressure of the nitrogen gas discharged from the pressure storage tank, and discharging the regulated nitrogen gas to an injection pipe installed on the side of a seat tightness and pressure setting test bench through a high-pressure hose; and the seat tightness and pressure setting test bench injecting the nitrogen gas injected from the pressure regulating valve into a lower portion of the pressurizer safety valve seated on an upper surface thereof to be able to measure an amount of the nitrogen gas for the seat tightness test and an amount of the nitrogen gas for the pressure setting test in the pressurizer safety valve.
The safety valve testing apparatus disclosed in Korean Patent No. 10-0311775 is designed to test abnormality of a safety valve using forcibly provided pressure. The safety valve testing apparatus includes a high-pressure cleaner supplying pressurized water; a tester including a pressure tank installed in an upright posture, an upper space of which enters a high-pressure state so as to buffer a change in pressure when receiving the pressurized water from the high-pressure cleaner at a lower portion thereof, a main valve preventing the pressurized water from simultaneously flowing to both a test connector connected with a subject safety valve and a recorder recording a test pressure state until set pressure is maintained in the pressure tank, and a flange connected with the subject safety valve through the test connector; and the recorder converting a change in output pressure of the main valve of the tester into an electrical signal, and recording the converted electrical signal. The pressure tank, to which the pressure is provided through the high-pressure cleaner, is additionally provided with an accumulator at a front end thereof which outputs pressure under overpressure. The flange is installed on a support plate. The support plate is configured such that a size thereof can be selected according to a size of the subject safety valve.
The method and system for testing safety relief valves disclosed in U.S. Pat. No. 4,893,494 utilizes a pressure vessel coupled with and being in communication with the subject safety relief valve and a high pressure fluid reservoir supplying the fluid to the pressure vessel. The method and system permit the subject safety relief valve with air as well as water. The system includes: a first pressure vessel; a second pressure vessel; means for releasably securing a safety relief valve and in fluid communication with the second pressure vessel; first means for controlling high pressure fluid flow into the first pressure vessel; a first fluid passageway connecting and disposed between the first and second pressure vessels; a second fluid passageway connecting and disposed between the releasable securing means and the second pressure vessel; and second means for controlling fluid flow between the first and second pressure vessels.
The bench test using the bench set is used for the set pressure test and seat tightness test for the safety valve at room temperature, but has conditions different from high-temperature thermal equilibrium conditions on which the safety valve is actually installed and operated on the facility. Thus, there is a problem in that, in the event of overpressure of the facility or system (e.g. reactor coolant system), an actual opening pressure of the safety valve is not matched with the set pressure of the safety valve based on the bench test.
The test facility test using the off-site test facility is performed in the state in which the subject safety valve is mounted on the test facility and under the same conditions of fluid, temperature, pressure and thermal equilibrium as the facility in which the safety valve is used. Thus, the test facility test using the off-site test facility is the most accurate test method among the aforementioned test methods. The apparatus for testing the performance of a pressurizer safety valve according to the present invention belongs to the off-site test facility, and can carry out the test under the same conditions on which the pressurizer safety valve is installed and operated.
FIG. 1 schematically illustrates a conventional apparatus for testing the performance of a safety valve.
The testing apparatus has a test vessel 1 supplied with steam pressure from a boiler 2 as a pressure supply. The boiler 2 must be equipped with accessories such as a water-supply facility 3 and a fuel injector. Since energy may be released during testing due to damage to a subject safety valve 4, the test vessel 1 is installed between the subject safety valve 4 and the boiler 2, and the boiler 2 is equipped with a pressure supply valve 7 and a bypass valve 8 of the pressure supply valve 7 to enable pressurization of the subject safety valve 4 up to pressure of the boiler 2 or to block the release of the energy when the subject safety valve 4 is damaged during testing. A shut-off valve 5 for the subject safety valve 4 is installed between the subject safety valve 4 and the test vessel 1 to shut off the steam in the event of the damage or leakage of the safety valve. This shut-off valve 5 has a sufficient volume that a flow of test fluid injected into the subject safety valve 4 from the test vessel is not restricted. A pipe connecting the two valves 5 and 6 has a sufficient size such that unnecessary pressure drop does not take place between the subject safety valve 4 and the test vessel 1. The test facility performing all flow tests on the safety valve is designed so that all of the valves, adaptors, flanges, and test nozzles withstand a discharge force of the subject safety valve 4 as well as the discharge force transmitted to the test vessel. All pressure sensing pipes are connected spaced apart from inlet and outlet nozzles 12 and 13 of the test vessel 1 in order to avoid an error in pressure measurement which is responsible for a flow rate during testing. In the case of the test using steam, the test facility is equipped with a steam trap 10 and a drain valve 11 so as to be kept warm by a heat insulator and to produce saturated steam of at least 98%.
In the conventional off-site test facility as illustrated in FIG. 1, the flow rate passing through the subject safety valve 4 and the overpressure applied to the subject safety valve 4 are involved in capacity of the boiler 2, i.e. the pressure supply, to generate flow. Thus, in order to test the safety valve having high set pressure and flow rate, a high-capacity boiler and accessories compatible with the boiler are required. As such, the establishment of the safety valve test facility requires relatively high cost and wide space. Further, when a discharge port of the subject safety valve is exposed to the air, excessive noise (e.g. of 130 dB or more) is generated. Thus, it is difficult to install the test facility near a residential area.